Use ?= for MK_DIR in firmware.mk
Use ?= for MK_DIR in firmware.mk

APU requires the use of ?= for MK_DIR but the use of lastword was
causing MK_DIR to be a subdir of makefiles. Changed lastword to
firstword (which is always the path to firmware.mk) which fixed
the problem.

Signed-off-by: Mark Charlebois <f1891e7fc976ae1bd489627e2954fb730751d728@gmail.com>

PX4/Firmware,Aerotenna/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,jlecoeur/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,darknight-007/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,darknight-007/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,PX4/Firmware,mcgill-robotics/Firmware,darknight-007/Firmware,Aerotenna/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,dagar/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,dagar/Firmware,darknight-007/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,dagar/Firmware,jlecoeur/Firmware,PX4/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,PX4/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware